Foamed plastics are plastics having reduced apparent densities due to the presence of numerous cells disposed throughout the mass of the polymer. Rigid foams usually produced at greater than about 320 kg/m3 density are known as structural foams, and are well known in the art. Structural foams are commonly used in various aspects of manufacturing molded articles in which low density polymer materials are desirable. Cellular polymers and plastics are made by a variety of methods having the basic steps of cell initiation, cell growth and cell stabilization. Structural foams having an integral skin cellular core and a high strength to weight ratio are made by several processes, including injection molding and extrusion molding, wherein a particular process is selected based upon product requirements.
Injection molding of structural foams is usually conducted under either low pressure or high pressure conditions. For example, during the injection molding process, a chemical blowing agent is typically introduced to the polymer resin melt in the extrusion barrel of an injection molding machine. The temperature of the extrusion barrel is increased under pressure, after which the pressure is released, injecting the polymer into a mold, permitting the chemical blowing agent to generate gas within the polymer. The expansion of the blowing agent pushes molten polymer material against the walls of the mold such that the material in contact with the walls has a higher density than the material toward the middle of the molded article. This establishes a density gradient wherein the outer surface areas of an injection molded article have a greater density than the core of the part due to more foaming in the center of the article. Thus, a gradient is established having smaller cells present near the mold surface with increasingly larger cells present toward the center of the article.
The use of blowing agents permits short shooting during the molding process. That is, because the blowing agent increases the volume of the expanding polymer composition, the mold is filled with less resin material than would be required without a blowing agent. Consequently, the density of the molded article may be reduced by about 10% to about 20% over articles molded without an incorporated blowing agent. Use of less polymer resin has the advantage of decreasing the weight of the final molded product.
Initiation of cell formation and promotion of cells of a given size are controlled by nucleation agents included in the polymer composition. The nature of cell-control agents added to the polymer compositions influence the mechanical stability of the foamed structure by changing the physical properties of the plastic phase and by creating discontinuities in the plastic phase which allows the blowing agent to diffuse from the cells to the surrounding material. Typically, the resulting cells provide for a lightweight molded article, but do so at the expense of impact resistance. For example, nucleation agents often promote crystalline structures within the cooled polymer, which reduce impact resistance. Mineral fillers may be added to provide a large number of nucleation sites, but such fillers tend to serve as stress concentrators, promoting crack formation and decreasing the impact resistance of molded articles.
Poor impact resistance of structural foam articles may be improved by the inclusion of glass fibers in the polymer melt during processing. However, glass fibers are generally too large to substantially reinforce the foam cells formed by the bubble structures. Glass fibers are often coated with sizing agents, which may induce clumping and impair even dispersion of the fibers. In addition, the amount of glass fibers required to achieve reasonable impact resistance of structural foam increases the specific gravity of polymer used therein, thereby increasing the density of the foamed article. This defeats the purpose of using lightweight foamed articles in the manufacture of, for example, automobiles, where lightweight components are highly desirable. Consequently, the levels of glass fibers in polymer compositions for foamed articles are kept relatively low, meaning impact resistance of the molded products is poor.
Typically, the reduced strength of structural foams may be at least partially offset by increasing the wall thickness of molded articles. Increasing wall thickness requires more raw materials per unit molded, thereby increasing the cost of production.
U.S. Pat. No. 5,753,717 to Sanyasi discloses a method of producing foamed plastics with enhanced physical strength. The structural foams of Sanyasi utilize CO2 in combination with an adjustment in the extrusion temperature of molten polystyrene resins to improve foam strength. This process, however, does not improve the foam strength of other types of resins, and is not suitable for enhancing the strength of articles for use in, for example, automotive trim.
Structural foam automotive parts historically have inconsistent surface appearances due to variations in the density of the polymer near the skin or surface of these molded articles. The imperfections in the surfaces of molded structural foam articles usually limits the usage of these foam products to non-appearance (e.g., hidden or non-visible) parts or parts in which the surface has been textured. Examples of these structural automotive interior trim products include interior door panel structural members, instrument panel retainers, interior seat backs covered with fabric, load floors in the storage compartments of vehicles, side wall trim and the like. Some pickup truck beds can be made from structural foam. All of these products require reduced density and good impact resistance.